The construction of somatic cell hybrids between cell lines derived from several different mammalian species has permitted the detection, characterization and genetic localization of a number of cellular genes which participate in endogenous type C virus expression. The genes fall into three classes viz. (1) endogenous retravirus DNA sequences in normal DNA, (2) chromosomal integration sites for proviral DNA, (3) restriction genes which delimit replication of retrovirus in heterologous cell types. Viral and genetic characterization of two loci Bvr-1 and Bevi is advanced and the analysis of eight additional genes is in progress. Unequivocal genetic signatures of human and mouse cell lines have been developed using allozyme genotypes for purposes of cell contamination and transformation monitors. Quantitative analysis of viral expression and kinetics of synthesis of cell surface p30 and gp70 on retrovirus infected cells has been achieved. BIBLIOGRAPHIC REFERENCES: O'Brien, S.J., Kleiner, G., Olson, R. and Shannon, J.: Enzyme polymorphisms as genetic signatures in human cell cultures. Science 195: 1345-1348, 1977. O'Brien, S.J. and Boone, C.W.: Expression of feline leukemia virus antigens on cat lymphoma cells: Kinetics of biosynthesis. J. Gen. Virol., 1977 (In press).